1. Field of the Invention
This invention relates to an electric power detecting circuit and a gain control circuit using the same. More particularly, this invention relates to the construction of a circuit which measures the level of input electric power of an input amplifier of a modem in a communication system for setting up the modem and also for attaining automatic gain control of the input amplifier.
2. Description of the Prior Art
In an apparatus such as a modem incorporated in a data transmission system, its automatic gain control circuit is required to follow the level of input electric power or the operation of the modem is controlled depending on the level of input electric power. In such an apparatus, it is necessary to accurately and quickly detect the electric power level of the input signal so that the input signal level can be controlled to be constant.
Especially, when an analogue input signal is passed through a variable gain amplifier and is then converted into a digital signal to be processed subsequently by a digital signal processing circuit, it is necessary that the digital signal has a stable and accurate digital level.
A gain control circuit which meets the above requirement is proposed in Japanese Patent Application Laid-open No. 55-97714 entited "HYBRID GAIN CONTROL CIRCUIT" and laid open on July 25, 1980. The proposed gain control circuit is composed of an analogue gain control part making rough gain control and a digital gain control part making fine gain control, and the output of the gain control circuit is used to control the analogue and digital gain control parts.
However, the hybrid gain control circuit proposed in the above application is complex in structure due to the fact that it is composed of the analogue gain control part and digital gain control part. Further, the manner of control in the analogue gain control part making rough gain control in the proposed circuit is such that the gain of the variable gain amplifier is controlled on the basis of the level of the signal indicative of the difference between the amplifier output level and a predetermined reference level. Consequently, a variation in the level of the input signal leads to such a problem that a certain length of time is required until the amplification degree of the analogue gain control part is stabilized to be maintained constant.